Pipe welding involves joining the longitudinal ends of generally cylindrical pipe sections to form an elongated pipeline structure with an interior suitable for transporting gases or liquids. Automated pipe welding techniques are sometimes employed, such as the use of welding apparatus having two welding bugs which continuously move on a track around the periphery of the pipe to form the root bead, as shown in Parker U.S. Pat. No. 5,676,857. However, manual stick welding remains the primary technique used in welding pipe sections together to form pipelines, wherein cellulosic and vertical down low hydrogen stick electrodes are commonly employed for these applications. FIGS. 1 and 2A-2E show two pipe sections 1 and 2 being welded together for constructing a pipeline 10, wherein ends 1a and 2a of the sections 1 and 2 are welded together using conventional stick welding procedures. As best shown in FIG. 2A, the ends 1a, 2a are each machined to provide an outwardly facing external bevel 3 and a narrow flat land 4. The pipe sections 1 and 2 are then positioned in axial alignment with the ends 1a, 2a proximate one another using some form of clamping arrangement (not shown) in a closely spaced relationship to provide a narrow gap of length 5 between the two lands 4 with the beveled surfaces 3 forming a weld groove 6 having a groove angle 7, typically about 60 degrees. In practice, the ends 1a, 2a may initially be brought into contact with one another, and then the pipe sections 1 and 2 are carefully separated to provide the desired gap distance 5. As shown in FIG. 2B, the pipe ends 1a and 2a are then joined using an initial root pass to form a root bead 8 to fill the gap between the land edges 4. After the root pass, one or more stick weld filler passes are performed in FIGS. 2C-2E using a stick electrode 9 connected to a source of electrical welding current (not shown) to form a welding arc 15 between the electrode 9 and the pipe material. FIG. 2C illustrates a first filler pass in which a first filler bead or weld layer 11 is created, after which second, third, and fourth filler welds 12, 13, and 14, respectively, are created in similar fashion using stick welding (FIGS. 2D and 2E) to fill the groove 6, where the number of fill passes varies with the pipe thickness. A cap pass weld is then formed just prior to the weld being flush with the outer surface of the pipe so that the weld metal following the cap pass is at least flush with the outer surfaces of the pipe sections 1 and 2.
As shown in FIG. 1, the filler welds are performed in two operations, each beginning at the top of the joined pipe sections and proceeding down and around in the directions shown by arrows 16. The welding position varies in each fill pass from down-hand welding, to vertical down welding, and eventually to overhead welding as the fill pass proceeds around each half of the pipe joint circumference. Because the stick welding operation leaves residual slag over each filler pass, a time consuming cleaning operation is required to remove the slag by brushing, chipping, chiseling, and/or other cleaning steps before proceeding with the next filler pass. It is therefore desirable to reduce the number of filler passes required for completely filling the grove at the pipe section ends to reduce the time needed to weld each new pipe section added to the pipeline 10. In this regard, stick electrode selection is an important consideration in pipe welding, where some stick electrodes allow slower welding and hence more material deposition per unit length in each fill pass than others for a given set of welding parameters (welding current, groove dimensions, pipe material, etc.). The volume of weld material that can be deposited in a given groove per unit length of joint in a single stick welding pass is sometimes referred to as stackability. Stick electrodes having good stackability performance allow the use of fewer filler passes for a given pipe size and weld groove size, wherein stackability performance may vary widely among stick welding electrodes from different manufacturers. Consequently, there is a need for techniques by which the stackability performance of a stick electrode can be quantified to facilitate informed selection of electrodes for pipe welding operations.